A new study of US prisons has found that numbers of people in jail are at an all-time high, with more than 1% of the adult population behind bars.
The Pew Center report calls the US the global leader in the rate at which it imprisons its citizens.
Over 2.3 million people were being held this year, it said - far ahead of other countries with large prison populations like China, Russia and Iran.
The report called for fewer low-risk offenders to be sent to jail.
It claims that the growing prison population "is saddling cash-strapped states with soaring costs they can ill afford, and failing to have a clear impact either on recidivism or overall crime".
With 750 inmates per 100,000 people, imprisonment cost the 50 states more than $49bn last year, up from less than $11bn 20 years earlier.
The rate of increase for prison costs was found to be six times greater than for higher education spending.
The US prison population almost trebled between 1987 and 2007